Power systems are going through a paradigm change. At the moment, the frequency of a power system is controlled by regulating a small number of large synchronous generators and most loads do not take part in the frequency control of the system. But now, the landscape of power systems is rapidly changing. Various non-synchronous distributed energy resources (DER), including renewables, electric vehicles and energy storage systems, are being connected to power systems. Moreover, most loads that do not take part in frequency control now are expected to take part in frequency control in the future. Hence, the number of active players to take part in frequency control in the future could easily reach millions, which imposes unprecedented challenges to the frequency stability of future power systems. The fundamental challenge behind this paradigm change is that future power systems will be power electronics-based, instead of electric machines-based, with millions of relatively small, non-synchronous and incompatible players. For example, on the supply side, most DERs are connected to power systems through power electronic converters. In transmission and distribution networks, many power electronic converters, such as HVDC links and FACTS devices, are being introduced to electronically control power systems in order to improve efficiency and controllability. On the load side, most loads will be connected to the grid through power electronic converters as well. For example, motors, which consume over 50% of electricity, are much more efficient when equipped with motor drives; Internet devices, which consume over 10% of electricity, have front-end power electronic converters; lighting devices, which consume about 20% of electricity, are being replaced with LED lights, which have front-end power electronic converters as well. The integration of power electronic converters into the electrical grid for distributed generation (DG), often by means of microgrids, has been a topic of intensive research and development.
Most of the converters nowadays are controlled to behave as current sources, through controlling the current exchanged with the grid. However, this makes them incompatible with the power systems, which are dominated by voltage sources.
Power electronic converters could play a similar role as synchronous generators in conventional power systems. Hence, it is reasonable to adopt some of the well established concepts and principles in power systems to control power electronic converters. Recently, it has been shown by the inventor of this disclosure that the well-known droop control strategy structurally resembles the widely-used phase-locked loop and that the conventional synchronous machines resemble the phase-locked loop as well. What is common among these different concepts is their intrinsic synchronization mechanism. The inventor of this disclosure has also shown that this synchronization mechanism should and could be adopted to build future power systems that are dominated by power electronic converters.